Radio frequency matching devices



Aug. 20, 1957 Filed April 1, 1 953 J. H. BRYANT 2,803,777

RADIO FREQUENCY MATCHING DEVICES 3 Sheets-Sheet Ill ELECTRON GUNINVENTOR JOHN H. BRYANT A 4 AZORIi Allg- 1957 J. H. BRYANT 2,803,777

RADIO FREQUENCY MATCHING DEVICES 7 Filed April 1, 1953 3 Sheets-Sheet 2FIG. 3

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INVENTOR JOHN H. ER YA IV T ATTORNEY 1957 J. H. BRYANT 2,803,777

' RADIO FREQUENCY MATCHING DEVICES Filed April 1, 1953 5 Sheets-Sheet 5COLLECTOR FIG. 6a

COUPL/NG BETWEEN HE]. ICES- AXIAL POSITION FIG. 7

INVENTOR JOHN H. ER YA/VT BY I? ATTORNEY This invention relates to radiofrequency matching States Patent devices and more particularly to meansfor obtaining a broadband match between radio frequency transmissionlines andhelical wave propagating structure of the type employed intraveling wave electron discharge. devices and delay lines.

The traveling wave type of tube is particularly useful in broadbandmicrowave systems since it is capable of amplifying radio frequencyenergy over an unusually broad band of frequencies. The tube includes aform of transmission line, usually a helix, for transmission ofmicrowave energy for interaction with an electron beam closelyassociated with the line. The helical characteristic of the transmissionline is such that the axial velocity'of microwave signals conductedalong the helical path is approximately the same as or slightly slowerthan the velocity of the electrons of the beam whereby the electricalfield of the microwave signals interact with the electron beam foramplification of the microwave signals.

One of the major problems in the helix type of traveling wave tube is inobtaining a broadband impedance match between radio frequencytransmission lines and the helical wave propagating structure. A helixtype of traveling wave tube may have a useful operating frequency rangeof over two to one, with the useful range limited by the quality of theradio frequency match. It is known that to cover a two to one or greaterfrequency operating range, coaxial line radio frequency circuitconnections are usually resorted to, since waveguide transmission lineshave well known bandwidth limitations. The standard impedance of coaxiallinesis normally of the order of 50 ohms, while the circuit impedance oftraveling wave tube helices may be several hundred ohms. There is then,clearly, the problem of matchinga low impedance coaxial radio frequencyline to a helical radio frequency line of a much higher impedance thanthe coaxial line. Furthermore, it is well known that a conductor inclose proximity to a conducting surface presents a surge impedance ofthe order of 50 ohms.

An object of this invention is the provision of means to provide auseful operating frequency range for a helix type of traveling wave tubeof over two to one.

Another object of this invention is to provide means for matching a lowimpedance radio frequency line to a high impedance helical radiofrequency line.

A feature of this invention is the provision of a transfer transmissionline including a conducting cylinder in a given spaced relationship withthe helical propagating structure wherein a portion thereof includes atapered relationship between said propagating structure and saidconducting cylinder, and a coupling helix disposed coaxially of saidcylinder and tapered surface to provide an impedance match betweenaradio frequency transmission line, to which said helix is connected,and said propagating structure.

Another feature of this invention is the provision of a transfertransmission line including an outer conductor tapered to cooperate withthe helical propagating structure in'a manner to effectively reduce theimpedance of said propagating structure to substantially 50 ohms and anextension of said outer conductor consistently spaced from saidpropagating structure to present a 50'ohm impedanc'e for connection to a50 ohm radio frequency transmission line.

Still another feature of this invention is the cooperation of an outerconductor with a tapered helical propagating structure arranged in apredetermined manner with respect to a radio frequency transmission linefor achievement of the desired impedance match.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional view of a traveling wave electrondischarge device illustrating an embodiment following the principles ofthis invention;

Fig.2 is a cross-sectional viewtaken along line '22 of Fig. 11; i

Fig. 3 is a graphical representation of the effect of the surroundingcoaxial cylinder upon the velocityof'propa- .gation as a function offrequency;

Fig.-4 is a graphical representation of the-effect 0f the coaxialconducting cylinder in the reduction of the helical impedance;

Fig. 5 is a longitudinal cross-sectional view of another embodiment forachieving radio frequency matchingbetween 21 radio frequencytransmissionline and a. helical Wave structure without actual physicalconnection therebetween;

Figs. 6 and 6A diagrammatically illustrate still another embodiment ofobtaining desired radio frequency matching following the principles ofthis invention; and

Fig. 7 is a longitudinal cross'sectional View illustrating anotherembodiment of this invention.

Referring to Fig. 1, there is illustrated a traveling wave electrondischargedevice comprising essentially an outer non-magnetic metallicconductive envelope or shell 1 surrounding a helical type wavepropagating structure 2. At one end of envelope 1 is located a suitableelectron gun 3 for furnishing a concentrated stream of electrons whichpass through the interior of helical conductor 2 for collection at acollector electrode 4 disposed in the opposite end of envelope 1. Theelectron gun 3 and collector 4 may take any of the known configurationshaving electrical properties suitable for this application which areknown in the art. The various electrode potentials and means forapplying them have received extensive treatment in the prior art and asa result are omitted in the drawings of this application to simplify thefigures thereof to more clearly illustrate the structural details forachieving the desired radio frequency matching in accordance with theprinciples of this invention. The traveling wave electron dischargedevice further is shown to have associated therewith a means 5 forproducing a magnetic field whose lines of flux are parallel to thelongitudinal axis of the tube for focusing and confining the electronbeam parallel to the axis of the tube. Means 5 may be a solenoid soconfigured that the desired magnetic field is developed, or it maycomprise a permanent magnet type electron-optical device for developmentof the desired axial magnetic field.

As hereinabove mentioned the characteristic impedance of helix 2 is ofthe order of several hundred. ohms while radio frequency transmissionlines of the wideband type, namely coaxial lines, may be of the order of50 ohms. Therefore, it is necessary to provide an input matching section6 capable of matching the 50 ohm transmission line 7 to helicalpropagation structure 2 and. likewise an output matching section 8 tomatch the output transmission line 9 to propagating structure 2. Forsuch a matching arrangement to be effective, it should cooperate in theefficient transfer of energy for amplification from transmission line 7to transmission line 9 without any appreciable loss of energy due tomismatch.

To assure a vacuum tight envelope 1, it is desirous to provide in theinput and output lines 7 and 9, glass beads 10 and 11, respectively.

Matching section 6, substantially identical to matching section 8, isshown to comprise a cylindrical coaxial portion 12 of envelope 1 in apredetermined spaced relationship with a circular conducting or helicalwound wire to form a portion of the transfer transmission line having acharacteristic impedance of the order of 50 ohms. It can be shownmathematically that the impedance of a transmission line consisting of acircular conducting Wire, such as the wire employed in forming helix 2,above an infinite plane conductor is given by .where h is the height ofthe wire center above the conducting plane, b is the radius of the wire,6 is the dielectric constant of the space between said wire and saidconductor and a is the permeability of that space. For practicalpurposes it has been found that this expression holds for surfaces ofrelative large radius of curvature. To properly meet the requirementsestablished by the above equation the last turn of helix 2 should bebrought out tangent to the helix diameter and joined to the centerconductor 13 of the coaxial radio frequency line 7 as indicated in Fig.2. The combination of conducting surface 12 and the proximity of thehelical conductor 2 provides an impedance looking into transmission line7 that can be made substantially equal to the characteristic impedanceof transmission line 7 over an extremely wide frequency range. It hasbeen ascertained that a radio frequency match with a maximum voltagestanding Wave ratio (V. S. W. R.) of 1.5 may be obtained for a frequencyrange of 300 to 1200 megacycles.

Thus, it is seen that the 50 ohm transmission line 7 can be matched to a50 ohm transmission line comprising conducting portion 12 in closeproximity to structure 2.

However, to achieve the advantages of the high impedance helicalstructure 2 for amplification of the radio frequency energy presentedthereto from line 7, it would be desirable to move the conductingcylinder 12 away from the active helix 2 known in this portion as thecoupling helical conductor 2a as soon as possible after the inputconnectron 1s made between helical conductor 2a and transmis- S1011 line7. To accomplish this envelope 1 is flared out at tapered portion 14 togradually increase the characteristic impedance of helix 2 in itsrelationship with conductmg cylmder 1. The length of the inputconnecting section 12 and tapered portion 14 may be in association withhelix 2a, referred to as the transfer transmission line, shown to have apredetermined ratio for obtaining a broadband radio frequency match. Ithas been ascertained that with dimension A equal to 1 inch and dimensionB equal to 2 inches that a match Within 1.5 V. S. W. R. can be obtainedfrom 450 megacycles to 1200 megacycles with a helix having a phasevelocity corresponding to the velocity of 6500 volt electrons.Therefore, it is possible to match 50 ohm transmission line 7 to anactive helical structure 2 employing only 3 inches of matching sectionsimilar to matching section 6 at frequencies ranging from 450 to 1200megacycles. It will be found that the 3 inch section 6 provides a radiofrequency match good for a lower frequency than required for aparticular application, therefore, it is possible to reduce thesedimensions to the extent that the desired match is obtained over onlythat portion of frequency bandwidth which is desirable. Such a reductionof the matching section or transfer transmission line provides a desiredradio frequency match between the transmissionlines 7 and 9 and thehelical structure 2. It has further been discovered that conductor 1tends to eliminate the dispersive character of the helical transmissionline 2. This dispersion characteristic is most pronounced at the lowfrequency end of the band and is extremely important when it is desiredto operate the traveling wave tube over a frequency range of two to oneor greater. Fig. 3 illustrates a plot of the effect of the surroundingcoaxial cylinder 1 in flattening out the velocity of propagation on thehelix 2 as a function of frequency.

The parameters of Fig. 3 are a quantity nearly equal to the ratio of theaxial velocity of a radio wave on a helix in a conducting cylinder tothe velocity the wave would have if it were traveling at the speed oflight along the direction of conduction, and flea cot. 1/, a quantityproportional to frequency, where \pzthe pitch angle of the helix,

ai=the helix mean radius, b=conducting cylinder radius, =the x/fi floo=the velocity of light propagation, f=the frequency of the radio wavepropagated on the helix, and v=the velocity of propagation of the radiowave. Thus the curves 14, 15, 16, and 17 indicate that the dispersivetendency of the helical structure 2 may be controlled by setting theinside diameter of the coaxial conducting cylinder with respect to thehelical diameter.

Fig. 4 illustrates the effect of the outer coaxial conducting cylinder 1upon reducing the helical impedance where the parameters are F('ya)which represents a factor in the impedance function of a helixsurrounded by a coaxial conducting cylinder where Ez is the peakelectric field on the axis with power P flowing, and 'ya represents aquantity substantially proportional to frequency, where y=the radialpropagation constant, w=the radius of the helix and b=the radius of theconducting cylinder. Curves 18, 19, 20, 21 indicate the effect of theouter conducting cylinder upon the helix impedance for the sameconditions of helix and conducting cylinder radius as represented by thecurves 14, 15, 16, and 17 respectively of Fig. 3.

It will be obvious to one skilled in the art that a compromise must bereached between the helix impedance reduction and the reduction ofdispersion by incorporating the radio frequency matching device inaccordance with this invention such that the overall performance of atube may be greatly improved by establishing the diameter of the coaxialcylinder at an appropriate value.

Not withstanding the importance of the dispersion of the radio wave byhelical structure 2 the reduction of helical impedance plays anextremely important part in achieving a radio frequency match between alow impedance transmission line and the relatively large impedancehelical line. The low impedance of the helical line means that more ofthe power in the radio propagating section of a traveling wave tube isflowing between the helical structure 2 and the conducting cylinder 1than was flowing outside the helix when the conductor was absent or isremoved from the proximity of the helix. Consequently, the presence ofthe outer conducting cylinder 1 has decreased the power flowing insidethe helix. There- ,inder 1 is a ing inside the helix with reference tothe total power flow- .ing may be controlled. For an example, for ahelix in the power carried by he1ix25 closer to the outer helix 25 fore,it can be seen that the proximity of conducting cylmeans whereby .theamount of power. flowfree space with a radial propagating constant equalto 1.5 and a total power P flowing, about /2 of P isflowing insidehelical structure 2, whilethe remaining /2 of P is flowing outside thehelical structure. On the other hand, with a conducting cylinder lhavinga diameter 1.1 times the helical diameter results in only about ofthepower flowing inside the helix, .while the remainder of P flows betweenthe helical structure 2 and the cylinder 1; Therefore, it can be seenthat by tapering the diameter of either the cylinder 1 or the helicalstructure 2, the power division between the helix or between coaxialhelices may be controlled with respect to the distance along the axis.

Fig. 5 illustrates an embodiment where radio frequency power is to betransferred to. a helix22 u nder aradio frequency matched conditionwithin a conducting cylinder .or envelope 23. without actually makingaphysical connection with this helix 22. The power is fed bytransmission line.24 to an outer helix 25, or coupling helicalconductor. In region C the outercylinde r 23 is removed far enough fromhelix 25 such that the fields of the outer helix 25 are strong on itsaxis and hence will interact with the fields of the inner helix 22 in.such a manner that will be, transferred to helix 22. In. region D theouter cylinder is gradually' tapered suchthatthe outer helix essentiallyhas no interaction with the inner helix 22 since the power remaining onthehelix 25 will be located between the. outer conductor23 and this.helix 25. Helix 25 after encountering region D may be terminated at 26in a matchedload thereby providing structure incorporating thecharacteristics of a helix in association with a conducting cylinder toprovide a smoothtransition of power between an outer helix or couplinghelical conductor, and an inner coaxial helix for amplification ofthispower as is the normal behavior of a traveling wavetype device.

The principles hereinabove outlined in,connection with .Figs. 1, 2, and5 illustrate the. possibility of employing a conducting cylinderconfigured to cooperate with coaxial helices. for the extraction ofpower flowing on the inner one of said coaxial helices. Fig. 6.illustrates such an application wherein the outer conducting. cylinder27 is concentric. to an inner helix 28 and an outer coaxial helix 29, orcoupling helical conductor, employed to remove radio frequency powerflowing in either direction on inner helix 28. To provide areflectionless transmission, 3 it is desired to provide a tapering ofthe coupling between the helices 28and 29 along the axis. oftheconducting cylinder 27. This is accomplished by aproper tapering of thediameter of the outer cylinder 27 as indicated at portion 30 of theouter conductor 27 so that at the ends at the outer helix very littleinteraction occurs between the coaxial helices. However, the interactionat p the center is relatively strong and the power from the main orinner-helix "28 may be substantially removed therefrom as indicated inthe curve of Fig. 6A. The power removed may thenbe coupled througheither of the terminals 31 and 32 depending upon the direction of powerflow and the appropriate load into which it is desired to couple thispower. The employment of this embodiment will allow attenuation ofreflected waves without employing lossy material in contact with thepropagating structure wherein coupling helix 29 is disposed in theforward portion of a traveling wave tube in a manner to produce thedesired amplification therein.

In all of the embodiments of this invention a problem associated withsuch high power tubes is the power handling capabilities of the vacuumseals such as seals and 11 of Fig. 1 in the radio frequency transmissionlines. Consequently, in high power tubes it may be desirable totransform from a small diameter helical wire to a larger size coaxialline prior to encountering such a vacuum seal.

1 An embodiment of Fig. 7 illustrates a suitablearrangementfor'transforming from a small diameter helical radiofrequency line which provides one wound wire to a large size coaxialline in or near the ,output section of the helix.

Referring to Fig. 7, there is illustrated that the helical conductor 33in the last few turns thereof is tapered to a larger size as indicatedat 34. At the same time the main diameter of the helical wound wire istapered in a manner such that the helical conductor is moved into closerproximity with the, outer conductor 35 until the impedance of thetransfer transmissionline including the helical conductor 33 adjacent tothe outer conductor in region 34 has a characteristic impedance of theorder of 50 ohms. As the diameter of the helix is being increased thediameter of the conductor forming the helix may be increased until thediameter substantially approximates the diameter of the innerconductor36 of the radio frequency line37. ,Thislarger diameter-wire enables theemploymentof larger vacuum seals 38 in the more-realistic power handlingcapability for vacuum seals.

While I have described above the principles ofthis invention inconnection with specific apparatus, it is to be clearlyunderstood thatthis description is made onlyby way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims. I

.Iclaim: i 1. A broadband radiofrequency matching device for use betweena low impedance transmission line of the coaxial type and a highimpedance wave propagating structure ofthe helical type, comprising atransfer transmission line including a conducting cylinder and acoupling helical conductor disposed within and coaxially of saidcylinder, said transfer line being characterized by three sections, thefirst section thereof having a structural relationship in which aconstant narrow spacing is provided between said cylinder and saidcoupling helical conductor, for a longitudinal length equalapproximately to third the length of said transfer line to extend thefrequency range over which an impedance match is accomplished, thesecond section thereof having a structural relationship in which arelatively wide spacing is provided between said cylinder and saidcoupling helical conductor for, a predetermined longitudinal length ofsaid transfer line, and the third section thereof having a structuralrelationship in which the spacing between said cylinder and saidcoupling helical conductor is varied gradually between the wide andnarrow spacing of the first and second sections.

2. A device according to claim 1, wherein the cylindrical portion ofsaid third section is tapered to provide said variation in spacingbetween said cylinder and said coupling helical conductor.

3. A device according to claim 1, wherein the turns of said couplinghelical conductor of said third section is varied in diameter to providesaid variation in spacing between said cylinder and said couplinghelical conductor.

4. A device according to claim 3, wherein the diameter of the wireforming said coupling helical conductor is gradually increased toprovide direct coupling to large diameter coaxial transmission lines.

5; A device according to claim 1, wherein said propagating structureincludes a helical transmission line and the coupling helical conductorof said third section is connected to said helical transmission line asa continuation thereof.

6. A device according to claim 5, wherein the turns of said couplinghelical conductor of said third section is varied in diameter to providesaid variation in spacing between asid cylinder and said couplinghelical conductor.

7. A device according to claim 1, wherein said propagating structureincludes a helical transmission line and said coupling helical conductoris disposed in concentric relation to said helical transmission line.

8. A device according to claim 7, wherein the consaid variation inspacing between said cylinder and said coupling helical conductor.

' 9. A device according to claim 7, wherein said coupling helicalconductor is appropriately terminated at the end thereof for apredetermined matched coupling of energy between said propagatingstructure and said coupling helical conductor.

10. A device according to claim 1, wherein said propating structureincludes a helical transmission line and said coupling helical conductoris disposed in concentric relation to said helical transmission line anda second substantially identical transfer transmission line is disposedcoaxially of said helical transmission line in a manner whereby thegradual variation of the third section of said transfer line isdiametrically opposed to the gradual variation of the third section ofthe original transfer line.

11. A broadband radio frequency matching device for use between a lowimpedance transmission line of the coaxial type and a high impedancewave propagating structure of the helical type, comprising a transfertransmission line including a conducting cylinder and a coupling helicalconductor disposed Within and coaxially of said cylinder, said transferline being characterized by a three sections, the first section thereofhaving a structural relationship in which a constant narrow spacing isprovided between said cylinder and said coupling helical conductor for-alongitudinal length equal approximately to one third the length of saidtransfer line to extend the frequency range over which an impedancematch is accomplished and to present an impedance equal substantially tothe impedance of said coaxial line, the second section thereof having astructural relationship in which a relatively wide spacing is providedbetween said cylinder and said coupling helical conductor for apredetermined longitudinal length of said transfer line to present animpedance equal substantially to the impedance of said high impedancewave propagating structure, and the third section thereof having astructural relationship in which the spacing between said cylinder andsaid coupling helical conductor is .varied gradually between the wideand narrow spacing of the first and second sections to effectsubstantially reflectionless transfer of energy therebetween.

12. A broadband radio frequency matching device for use between a lowimpedance transmission line of the coaxial type and a high impedancewave propagating structure of the helical type, comprising first andsecond transfer transmission lines, each of said transfer transmissionlines including a conducting cylinder and a coupling helical conductordisposed within and coaxially of said cylinder, each of said transferlines being characterized by three sections, the first section thereofhaving a structural relationship in which a constant narrow spacing isprovided between said cylinder and said coupling helical conductor for apredetermined substantial longitudinal length of said transfer line toextend the frequency range over which an impedance match isaccomplished, the second section thereof having a structuralrelationship in which a wide spacing is provided between said cylinderand said coupling helical conductor for a predetermined longitudinallength of each of said transfer lines, and the third sectionthereofhaving a structural relationship in which the spacing betweensaid cylinder and said coupling helical conductor is variedgraduallybetween the wide i and narrow spacingof the first and second sections,said propagating structure including a helical transmission line andsaid coupling helical conductor of said second transfer line being acontinuation of said coupling helical conductor of said first transferline and being disposed-in concentric relation to said helicaltransmission line, said first and said second transfer lines beingdisposed in a manner whereby said second sections thereof are continuousand the gradual variations of said third sections are oppositelydisposed, whereby a tight coupling is established between saidcontinuous coupling helical conductor and said helical transmissionline. 7

13. A device according to claim 12, wherein said coupling helicalconductor is appropriately terminated at the ends thereof forpredetermined matched coupling of energy between said coupling helicalconductor and said propagating structure.

14. In a traveling wave electron discharge device including a vacuumhousing, a collector electrode at one end of said housing, means forproducing an electron beam for flow along a given path between said beamproducing means and said collector electrode at the other end of saidhousing and a radio frequency propagating structure disposed adjacentsaid path within said housing to enablereaction between the electrons ofsaid'beam and radio frequency energy propagated along said structure; acoupling section disposed along said path within said housingintermediate said beam producing means and said collector electrode andin coaxial coupling relation to said propagating structure to couplereflected wave energy in a refiectionless manner from said propagatingstructure.

15. In a device according to claim 14, wherein said coupling sectionincludes a helical coupling conductor concentric to said propagatingstructure for a predetermined length thereof and a termination at oneend of said helical coupling conductor to attenuate the reflected waveenergy coupled from said propagating structure.

-l6. In a device according to claim 15, wherein said coupling sectionincludes a conducting cylinder having a predetermined spacedrelationship with respect to said helical coupling conductor whereby atight coupling is established between said coupling conductor and saidpropagating structure.

17. In a device according to claim 14, wherein said coupling sectionincludes a helical coupling conductor and a conducting cylinderconcentric to said propagating structure whereby a predetermined'spacedrelationship between said coupling conductor and said cylinder providesa tight coupling between said coupling conductor and said propagatingstructure.

References Cited in the file of this patent UNITED STATES PATENTS Re.23,647 Lindenblad Apr. 21, 1953 2,516,944 Barnett Aug. 1, 1950 2,588,831I-Iansell Mar. 11, 1952 2,588,832 Hansell Mar. 11, 1952 2,615,141Hansell Oct. 21, 1952

